Printbars which are used in image recording systems are well known in the art. Such printbars are generally comprised of a linear array of discrete, light-emitting sources. Examples of printbars include wire dot, electrostatic, ink jet, and thermal printheads. Light emitting diode (LED) printbars are commonly used because of their high resolution, which is obtained by arranging a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor, and by selectively energizing the LEDs at the proper times, a desired latent electrostatic image can be produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, a word processor or some other source of digitized image data is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures. U.S. Pat. Nos. 4,689,694; 4,706,130; 5,138,337 and 5,126,759 are representative of prior art printhead control circuitry. Prior art exposure control systems are disclosed in U.S. Pat. Nos. 4,525,729, and 5,025,322. Those prior art references are hereby incorporated by reference.
To create high quality images using an LED printbar, each of the LEDs should output the same amount of light when activated. To meet current copy quality goals, the printbar light output uniformity must be within plus or minus 1 or 2%. It is known in the prior art to correct printbar LED outputs to this level during an initial calibration procedure. A correction matrix of light output values for each pixel is created and stored in a memory on the printbar. Those values are downloaded to correction circuitry each time the printer is to be used. The correction circuitry compensates for light output differences by controlling the electrical signals to the LEDs.
However, the LEDs may have different aging characteristics which will eventually result in pixel-to-pixel non-uniformity. To a first approximation, a decrease in an individual LED's light output is a simple function of the LED's accumulated on time. A prior art solution to the aging problem is to provide a photosensor on the printbar. That photosensor is then periodically scanned across the printbar as each pixel is individually turned on. The light intensity of each pixel is determined and, if necessary, the outputs from various LEDs are adjusted. While this system is beneficial in compensating for aging, it is rather expensive and uses valuable space near the photoreceptor. Other solutions to the aging problem are described in U.S. Pat. Nos. 5,016,027 and 4,982,203. However, those methods may not be optimal. Therefore, a new method of maintaining pixel-to-pixel exposure uniformity would be useful.